This invention relates to an optical disc device for recording information to or playing back information from an optical disc having concentric or spiral information tracks.
In recent years, optical disc devices are being developed for recording information to or playing back information from optical discs such as CDs (compact discs) or MDs (mini discs) that have concentric or spiral information tracks. The basic configuration of these optical disc devices is such that information tracks are tracked with a light spot generated by condensing a light beam emitted by a laser diode or the like with an objective lens and the information is recorded or played back. For this reason, in optical disc devices, the so-called tracking servo is a mandatory element. The tracking servo generates a tracking error signal that by some means or other indicates the positional discrepancy between an information track and the light spot, and moves the light spot until that signal becomes zero.
Various methods have been proposed as a method for generating the tracking error signal. One of those methods is the push-pull method which uses a divided light reception element. On the plus side, this method features a simple configuration, so that manufacturing costs are low, and, as compared to the three beam method, etc., it is not necessary to have a long light path length, so that the size can be made smaller. On the minus side, a direct current offset appears in the tracking error signal according to the position of the light spot on the light receiving element, which constitutes a shortcoming.
Recently, proposals have been made to implement a servo wherein the direct current offset is used as a spot position signal indicating the position of the light spot on the light receiving element in the optical head, and using that signal for controlling the position of the light spot, as seen in the optical disc device disclosed in Japanese Patent Application Laid-Open No. H9-274726/1997, for example. (Such a servo is here called a spot position servo). In the conventional optical disc device proposed here, peak signal detection is performed on the light detection output indicating each of the light reception quantities in a bifurcated light receiving element, and the spot position signal is generated by taking the differential between the two signal detection results.
The conventional optical disc device described above, as shown in FIG. 12, comprises a tracking error signal generator circuit 1201 for generating tracking error signals, a tracking servo circuit 1202 that uses the tracking error signal generated here to control an objective lens so that the light spot follows an information track on the optical disc, a spot position signal generator circuit 1203 having a configuration for taking the differential between the two signal detection results and generating the spot position signal, a spot position servo circuit 1204 that, based on the spot position signal here generated, controls the position of an objective lens so that the light spot is positioned in the center of the light receiving element, and a changeover switch 1206 for switching the signal supplied to a tracking actuator 1205 that moves an objective lens in a radial direction on the optical disc. Ordinarily, the configuration is made so that, when so-called fast access is performed, wherein tracking servo processing is performed by the tracking servo circuit 1202, and the optical head is moved in a radial direction, the spot position servo is activated by the spot position servo circuit 1204. The purpose of activating the spot position servo at the time of fast access is to prevent movement in the objective lens during fast access.
However, when an attempt is made to actually implement such an operation as this, there is no guarantee that the outputs of the tracking servo circuit 1202 and the spot position servo circuit 1204 will coincide at the time of switching.
In FIG. 13 is plotted the low-frequency component of a drive signal that is supplied to the tracking actuator 1205 when switching between tracking servo processing and spot position servo processing in a conventional optical disc device. In the time period 1301 wherein the tracking servo is engaged, the drive signal will change to a sine wave due to the eccentricity of the optical disc because the light spot follows the information track. When the spot position servo is started (time period 1302), in conjunction with starting fast access, etc., a step difference will occur at the time of starting. Subsequently, when the spot position servo reaches the target position, the drive signal converges to an output that is according to the target position. Possible causes of the step difference in the drive signal noted above are the difference in open loop gain between the tracking servo and the spot position servo, and differences in the direct current offset between the tracking error signal and the spot position signal.
In particular, when the low-frequency component does not coincide, a step difference develops in the tracking actuator drive signal at the time of switching, whereupon the objective lens gets moved at high speed. Also, when fast access is started wherein the number of crossed tracks is detected from the light reflected from the optical disc, the speed of movement of the optical head is low, wherefore the objective lens movement speed becomes faster than the optical head movement speed, whereupon errors occur in the detection of the number of moving tracks.
An object of the present invention, in view of the points raised above, is to provide an optical disc device that, by always causing an objective lens to move smoothly, effects stabilized access operation and tracking servo operation.
In order to attain that object, the optical disc device described in claim 1 comprises: an optical head comprising light spot movement arrangement for causing a light spot directed onto an optical disc to move in a radial direction of the optical disc; tracking error signal generator for generating a tracking error signal indicating the relative positions of the light spot and information tracks on the optical disc; spot position signal generator for generating a spot position signal indicating the radial direction position of the light spot on a light receiving element in the optical head; tracking servo means for generating a tracking drive signal for driving the light spot movement arrangement so that the light spot follows the information track, based on the tracking error signal; spot position servo means for generating a spot position drive signal for driving the light spot movement arrangement so that the position of the light spot on the optical head is fixed, based on the spot position signal; selector for selecting one or other of the tracking drive signal and the spot position drive signal and supplying the same as a selected drive signal to the light spot movement arrangement; and discontinuation cancellation means for implementing processing, when the tracking drive signal and the spot position drive signal are switched between, so that the low-frequency components of the selected drive signal before switching and the selected drive signal after switching do not become discontinuous.
Further, the optical disc device described in claim 2 is the optical disc device described in claim 1, comprising discontinuation cancellation means for performing at least one of two processes, namely a first process for setting an initial value in the spot position servo means based on the low-frequency component of the tracking drive signal, when the selector switch the selected drive signal from the tracking drive signal to the spot position drive signal, and a second process for setting an initial value in the tracking servo means based on the low-frequency component of the spot position drive signal, when the selector switch from the spot position drive signal to the tracking drive signal.
Further, the optical disc device described in claims 3 and 5 is the optical disc device described in claim 1, comprising discontinuation cancellation means for diminishing the gain in the spot position servo means, for a predetermined time period, or until the spot position signal declines to or below a predetermined value, when the selector switch the selected drive signal from the tracking drive signal to the spot position drive signal.
Further, the optical disc device described in claims 4 and 6 is the optical disc device described in claim 1, comprising discontinuation cancellation means for diminishing the gain in the spot position servo means, for a predetermined time period, or until the tracking error signal declines to or below a predetermined value, when the selector switch the selected drive signal from the spot position drive signal to the tracking drive signal.
Further, the optical disc device described in claims 7 is the optical disc device described in claim 1, comprising discontinuation cancellation means that internally comprise spot position signal memory means for storing spot position signals, processing means for performing processes on spot position signals stored by the spot position signal memory means, and spot position servo target setting means for setting servo targets for the spot position servo means, wherein, when the selector switch the selected drive signal from the tracking drive signal to the spot position drive signal, the spot position signal is stored by the spot position signal memory means, and the output of the processing means is made the servo target that is set by the spot position servo target setting means.
Further, the optical disc device described in claims 8 is the optical disc device described in claim 1, wherein the tracking servo means have means for extracting the low-frequency component of the tracking error signal, and the spot position servo means have means for extracting the low-frequency component of the spot position signal.
And the optical disc device described in claims 9 is the optical disc device described in claim 6, wherein the discontinuation cancellation means, when the tracking drive signal and the spot position signal are switched between, perform control to set the low-frequency component of the selected drive signal after switching so that the low-frequency component of the selected drive signal after switching becomes equal to the low-frequency component of the selected drive signal before switching.